I. Field
The following description relates generally to wireless communications, and more particularly to techniques for improved channel estimation.
II. Background
In the not too distant past mobile communication devices in general, and mobile telephones in particular, were luxury items only affordable to those with substantial income. Further, these mobile telephones were of substantial size, rendering them inconvenient for extended portability. For example, in contrast to today's mobile telephones (and other mobile communication devices), mobile telephones of the recent past could not be placed into a user's pocket or handbag without causing such user extreme discomfort. In addition to deficiencies associated with mobile telephones, wireless communications networks that provided services for such telephones were unreliable, covered insufficient geographical areas, were associated with inadequate bandwidth, and various other deficiencies.
In contrast to the above-described mobile telephones, mobile telephones and other devices that utilize wireless networks are now commonplace. Today's mobile telephones are extremely portable and inexpensive. For example, a typical modern mobile telephone can easily be placed in a handbag without a carrier thereof noticing existence of the telephone. Furthermore, wireless service providers often offer sophisticated mobile telephones at no cost to persons who subscribe to their wireless service. Numerous towers that transmit and/or relay wireless communications have been constructed over the last several years, thus providing wireless coverage to significant portions of the United States (as well as several other countries). Accordingly, millions (if not billions) of individuals own and utilize mobile telephones.
The aforementioned technological advancements are not limited solely to mobile telephones, as data other than voice data can be received and transmitted by devices equipped with wireless communication hardware and software. For instance, several major metropolitan areas have implemented or are planning to implement citywide wireless networks, thereby enabling devices with wireless capabilities to access a network (e.g., the Internet) and interact with data resident upon such network. Moreover, data can be exchanged between two or more devices by way of a wireless network. Given expected continuing advancement in technology, a number of users, devices, and data types exchanged wirelessly can be expected to continue to increase at a rapid rate. Due to such increase in use, however, networking protocols currently employed to transmit data are quickly becoming inadequate.
Orthogonal Frequency Division Modulation or Orthogonal Frequency Division Multiplexing (OFDM) is one exemplary protocol that is currently utilized in wireless environments to transmit and receive data. OFDM modulates digital information onto an analog carrier electromagnetic signal, and is utilized, for example, in IEEE 802.11a/g WLAN standard. An OFDM baseband signal (e.g., a subband) constitutes a number of orthogonal subcarriers, where each subcarrier independently transmits its own modulated data. Benefits of OFDM over other conventional wireless communication protocols include ease of filtering noise, ability to vary upstream and downstream speeds (which can be accomplished by way of allocating more or fewer carriers for each purpose), ability to mitigate effects of frequency-selective fading, etc.
In order to effectively communicate in a wireless environment, an accurate estimate of a physical (wireless) channel between a transmitter and receiver is typically needed. This estimation allows a receiver to obtain data delivered from a transmitter on various available subcarriers. Channel estimation is generally performed by delivering a pilot symbol to a receiver, wherein the pilot symbol is associated with modulation symbols known to such receiver. Accordingly, a channel response can be estimated as a ratio of a received pilot symbol over a transmitted pilot symbol for subcarriers utilized in pilot transmission. One exemplary conventional manner of obtaining a channel estimate is to assume a channel length (e.g., by utilizing a cyclic prefix), and thereafter analyze a number of observations in the frequency domain that relates to a number of observations required for adequate channel estimation in the temporal domain. More specifically, a defined number of pilot tones provide a number of observations of the channel in the frequency domain. Thereafter, a linear transformation can be applied to observations relating to the pilot tones in order to obtain corresponding observations in the temporal domain. In one particular example, an Inverse Fast Fourier Transform (IFFT) can be applied to observations relating to the pilot tones. Upon receiving the observations in the temporal domain, all such pilot observations can be averaged (with respect to each symbol instant upon the pilot carriers) to obtain an estimate of the physical channel.
In certain cases, the above-described channel estimation technique can lead to an irreducible noise floor that in turn can affect decoder performance. While this noise floor may not be significant enough to cause problems for most conventional data packets and/or modulating operations, it can cause performance degradation in the decoding of packets with high spectral efficiency (e.g., packet formats utilizing 64 QAM modulation that operate in conditions with high signal to noise ratio). Thus, conventional channel estimation systems and/or methodologies are frequently ineffective for such data packet formats.
In view of at least the above, there exists a need in the art for a system and/or methodology for mitigating flooring in connection with channel estimation given a high-level data packet.